Basic questions: buffers and impedance...

[pott]

I don't REALLY understand what a buffer does... Basically the way I see it, without a 3PDT (or DPDT without a LED), the signal still goes through the circuitry some. The buffer aims to reroute this signal or, boost it, or just stop it...? It's meant to drive long cables so I gather it IS some kind of boost?

Also... What is impedance?  I've been looking allover the place for a basic, clean, simple explanation but so far nothing but technical explanations which don't really help...
Just some simple basics would do, no need to dwelve into deep electronics. But all that would surely help me understand pedals a little better! Thanks guys!

[R.G.]

Well, you could read "Impedance - what is is and why is it doing all those terrible things to you." http://www.geofex.com/Article_Folders/impednc.htm

At GEO. http://www.geofex.com.

Impedance is just what it sounds like - how much does some part of the circuit impede the flow of electrical current. High impedance means it resists current flow a lot. It takes a big voltage to get a tiny current through a high impedance. Low impedance means it lets electrical current flow easily.

Impedance is just a way of saying "resistance" but for AC signals and with capacitance and inductance taken into account. That means it has to show how the circuit lets more or less current flow as frequency changes.

[R.G.]

But back to your first question.

All signal sources have some impedance themselves. For instance, if you short out a guitar pickup and pluck a string, the string still makes a signal voltage, but all of that voltage is expended across the resistance and inductance of the pickup itself. That internal impedance limits how much current can ever flow out of the pickup.

Then there's the issue of loading. A pickup's internal impedance is primarily the resistance of the winding wire at DC, maybe 5K to 10K ohms, plus the inductance of the coil. So if you put a low resistance across the output, maybe 10K, the signal is divided across the pickup's internal impedance and the external load. If you put a high impedance across the output, the high impedance will let very little current flow out of the pickup, and so all of the pickup's voltage appears at the high impedance, very little is lost to the internal impedance of the pickup itself.

So a high impedance input on a buffer lets essentially no current flow into the buffer - that preserves all of the signal voltage to be amplified by the buffer since the buffer does not load down whatever drives its input. And that is the whole point of a buffer - it makes life easy on a signal source like a guitar, and provides the muscle to drive loads because buffers normally have low output impedances, so they can move a lot of current on their outputs.

In the case of a guitar and a buffer, there is the issue of the guitar's internal impedance. Remember that pickups look like a resistance and an inductor in series with the signal source. The impedance of an inductor goes up with increasing frequency. So if there is even a high resistance load like maybe 100K, at some high frequency the impedance of the inductance is even bigger and the external load eats so much current that little of the signal voltage is left. This is frequency-selective, so the high frequencies - treble, brightness, sparkle - is lost to loading first.

A buffer only sips the tiniest amount of signal current, letting almost all of the signal voltage appear at the buffer input.The buffer then puts some muscle into shoving that signal on down the cable. It's like the buffer is your bodybuilder cousin doing the heavy lifting for you. Some buffers have voltage gain, but mostly what they have is current gain. A buffer may have a voltage gain of one or two, but a current gain of thousands. So you're right, there is a boost, but it's a current boost.

Cables look primarily like a capacitor. A capacitor has an impedance which decreases linearly as frequency goes up. Double the frequency, you get half the impedance, which means twice the current flow, all other things being equal. That's what makes it so hard for pickups to drive cables. A cable looks like a steadily decreasing impedance, eating every more current as frequency rises, and that's the hardest thing for an inductive pickup to cope with. A buffer holds off that capacitive cable from sucking all the current out of the pickup, and is tough enough to drive the cable even at very high frequencies.  That's why buffers are used to drive cables.

[PerroGrande]

Hi Pott,

R.G.'s comments are, as always, spot on.   

You rarely see an "easy" explanation of impedance, because the math behind it is reasonably complicated.  However, for purposes of a simple explanation, you can think of it as a fancy form of resistance.   Impedance takes into account two things: resistance and reactance.  Reactance is in the game only when alternating current (such as the waveforms created by your guitar pickup) is present.  Components such as capacitors and inductors introduce reactance into the works.  Resistors introduce resistance into the works.  The combination of the two is called impedance.  To make matters more complicated, reactance (and thusly impedance) is a frequency-dependent value.  For example, the tone control in your guitar takes advantage of this concept.   The good news is that Ohm's law applies to impedance (at a given frequency) the same way that it does to resistance.  The tone control then becomes essentially a frequency-dependent voltage-divider. 

As R.G. pointed out, impedance impedes... Sounds simple, but that is precisely what it does.   However, the implications of impedance are numerous and important to understand. 

In the world of effects, we speak about input and output impedance of a circuit. 

A circuit with a high input impedance will only "use up" a small amount of the input signal.   That is to say, the previous stage or device will be less likely to "notice" (i.e. be affected by) that something has been connected to it.   Remember -- impedance is just a fancy form of resistance and Ohm's law applies.  As impedance gets larger, it begins to appear more like an open circuit -- and, expectedly, less current flows. 

Conversely, a circuit with a low input impedance is "hungry".  It will "use up" more of the input signal.  As impedance gets smaller, it begins to appear more like a short circuit - and, expectedly, more current flows.   

The problem with "hungry" circuits is that the source may not be able to provide enough current, or that the characteristics of the source are changed as a result of the hungry circuit being attached.  This is often called "loading", incidentally.

In the case of guitars, a hungry circuit (low input impedance) will introduce loss -- treble frequencies first.  This is referred to as "sucking tone" from the guitar.  A pickup that normally sounds bright and crisp will sound muffled and muddy.  The solution to this is to connect your guitar to a circuit that is not hungry -- i.e. a high-impedance input.  This is one of the applications of a buffer.  A buffer circuit stands between your guitar (which is effected by loading) and a hungry circuit.  The buffer, unlike your guitar, has the "juice" to deal with a hungry circuit...

I'll write more a bit later...

[PerroGrande]

...a bit later...

So, as I mentioned in my previous message, a buffer can be used to present a high impedance initial stage to a guitar (or other device) to avoid loading down the source device or circuit.   This is good because it prevents a circuit from "sucking tone".

Another benefit to using buffers is that they can provide a voltage and/or current gain.   Typically, a buffer circuit that produces a voltage gain is called a "booster".  A voltage gain increases the amplitude of the signal (it amplifies) -- it makes it "louder" so to speak.  Most true buffer circuits (i.e. not boosters) produce near (or even slightly below) unity voltage gain.  They do not make the signal louder, per se, but they produce a gain in the output current.    This allows the buffer to "drive" lower impedance loads... 

When considering a typical connection from guitar to pedals to amplifier, there are numerous sources that produce a load on the input signal from your guitar pickups.  The cable introduces capacitance and some small resistance.  A given pedal may be "hungry" (lower impedance input).  True bypass switches add effective length to the cable, as does the cable from the last stompbox to the amp.  As you can see, there are numerous opportunities to lose signal (typically starting at the high frequencies and moving from there).  Buffers help to reduce these problems.

[pott]

You guys ROCK!! This actually makes a lot of sense.

More current = less risk of some of your signals getting lost in the tone sucking circuit. So basically your signal gets stronger and can resist the temptation 

And since pedals don't really care for current as much as voltage, it's easy to do without messing everything up?

So buffers are there to prevent pedals which have a natural low impedance (I'm assuming not all do?) to suck up some of the tones...
Since the pedal itself will most likely have some sort of opamp or transister stage in itself, I'm assuming that buffers in most gain pedals are either deactivated when the pedal is switched on, or simply have a then negligible effect on the sound?

[PerroGrande]

Pott,

Buffers reduce (and in some cases effectively eliminate) the effect of tone-sucking by long cables, pedals, etc. 

If the pedal circuit contains a buffer of reasonably high impedance, you can be sure that the buffer is on and operational when the pedal is active.  What the buffer does when the pedal is inactive varies.  If the pedal has a true (mechanical) bypass, then when the pedal is off, its buffer is out of the circuit. Other pedals actually keep the buffer in place when the pedal is deactivated (i.e. not stomped).  It just depends on the circuit.

Also -- do not assume that just because a pedal might have an op-amp or tranny at the input stage that it is a high-impedance pedal.  Depending on the way the input stage is set up, the impedance can actually be quite low (or even variable). 

Generally speaking, you can add a buffer -- especially a unity gain variety -- to a pedal with a high degree of safety.  You can also build them as independent boxes.  Buffers and/or boosters make a great project to start out with, too.  You can breadboard one fairly quickly and hear what its behavior will be prior to committing to a modification. 

[pott]

So basically a clean boost could have a low impedance and be problematic?

What's in a buffer circuit exactly...? Just something to convert the impedance, and maybe an optional clean amplification stage? I'll try to find schems.

Thanks for the clear ups so far!

[slacker]

There's a whole load of simple buffer schematics here http://www.muzique.com/lab/buffers.htm

[PerroGrande]

If a clean boost (or buffer) circuit is designed in such a way as to provide a relatively low input impedance, it is *possible* that tone could get sucked from a low-output pickup (such as a single coil pickup).  Most of them do not do this, however.

There are many, many ways to build buffers and/or boosters.  The most common methods involve one or more of the following:

MOSFET, FET, BJT, Op Amp. 

Check out the AMZ site for some simple but effective boosters and buffers.   Yeah -- what was just posted above 

[pott]

Just did thanks!

So basically... pickup = low impedance

The buffer transforms it into high impedance


BUT spits it out as low impedance again?

So I gather that pedals are happier with low impedance signals then, but that providing it directly from the pickup causes the tone sucking?

[PerroGrande]

Well, we didn't really get around to talking about *output* impedance yet...  I've only been speaking of the input side of the equation.

But for now, think of it like this:

Output from your pickups = WEAK signal. 

If this weak signal attempts to "drive" a hungry (low impedance) input, it will get SUCKED dry (loss of tone).

A buffer takes this weak signal and buffs it into a more durable, muscular signal that won't as easily get sucked dry by a hungry circuit...

[pott]

Alright thanks! I used to think that it was a boost, but now i see that it'd probably be better with unity gain 1 (and therefore using an opamp)

[slacker]

Here's a stupid way to think about it, it's not at all correct but it helps to understand it.

Imagine your pickups impedance is someone 6 feet tall and imagine that the pedal's input impedance is a door.
If the door is only 3 feet high (low impedance) then the person's going to struggle to get through it, which sucks (tone)
If the door is 10 feet high (high impedance) then the person fits though without any problems, so no tone sucking.

The buffer transforms it into high impedance

The buffer doesn't convert the impedance it's just that it's input impedance is so high compared to the pickup's impedance that it doesn't affect it.

BUT spits it out as low impedance again?

It spits it out as low impedance, but the buffer is capable of providing much more current than your pickup can so it's less affected by the impedance of things it might run into later on in the signal chain.
To go back to the dodgy door analogy, the buffer's output is like a tank, so if it hits a low door, it smashes right though it. So you don't get tone sucking or less of it.

[pott]

Awesome! Hehe right or not, I think right now I just need the very basics and that was exactly it! Thanks a lot!

One last question... The OUTPUT of most pedal... would it be high or low? I'm assuming high again, or would most actual circuits already take care of that?

[slacker]

Most pedals have a low output impedance but normally one that can provide a decent amount of current. That way when the signal hits the high impedance input of your amp or the next pedal in the chain you don't get tone sucking.

[PerroGrande]

Slacker -- you can call it dodgy, but I rather like your analogy.   

Pott, generally speaking the rule of thumb for circuit design is:

You want your input impedance to be high, so that your circuit doesn't load down previous stages. (i.e. it has a tall doorway)
(A low input impedance is the small doorway that sucks (tone) to get through)

You want your output impedance to be low, so that your circuit can source the most possible current. (it leaves like a tank).
(A high output impedance is a plastic wind-up toy that can't smash through much of anything)

There are exceptions to this -- especially in RF design where impedance matching is critical.  However, for most pedal circuits, I like to stick to this concept.

Hey -- while we're making analogies...

A low impedance input is like that annoying person that drinks all your beer (which sucks) and leaves little (or none) for you...
A low impedance output is a direct hose from the keg to you! 

[pott]

You know, as a student, this makes A LOT of sense hehe

Alright thanks so much guys, this surely helped me out quite a lot!